Field of the Invention
The present disclosure relates to a transducer that transmits and receives an elastic wave, such as an ultrasonic wave, and a measurement apparatus using the transducer. As used herein, the term “transmitting and receiving” refers to at least one of transmitting and receiving.
Description of the Related Art
To transmit and receive an ultrasonic wave, a capacitive micromachined ultrasonic transducer (CMUT), which is one type of capacitive ultrasonic transducer, has been developed (a CMUT is described in, for example, A. S. Ergun, Y. Huang, X. Zhuang, O. Oralkan, G. G. Yarahoglu, and B. T. Khuri-Yakub, “Capacitive micromachined ultrasonic transducers: fabrication technology,” Ultrasonics, Ferroelectrics and Frequency Control, IEEE Transactions on, vol. 52, no. 12, pp. 2242-2258, December 2005.) A CMUT is fabricated through a micro electro mechanical systems (MEMS) process using a semiconductor process.
FIG. 11 is a schematic cross-sectional view of a CMUT. As used herein, a set of a first electrode 102 and a second electrode 103 that face each other with a vibrating membrane 101 and a gap 105 therebetween is referred to as a “cell”. The vibrating membrane 101 is supported by a supporting portion 104 disposed on a substrate 200. A DC voltage generating unit 301 is connected to the first electrode 102, and a predetermined DC voltage Va is applied to the first electrode 102. The second electrode 103 is connected to a transmitting and receiving circuit 302. The second electrode 103 has a fixed potential that is close to a GND potential. In this manner, a potential difference Vbias (=Va−0 V) occurs between the first and second electrodes. By controlling Va, the value of Vbias is made equal to a desired potential difference determined by the mechanical property of the cell (several tens to several hundred volts).
If the transmitting and receiving circuit 302 applies an AC drive voltage to the second electrode 103, an AC electrostatic attraction force is generated between the first and second electrodes. Thus, the vibrating membrane 101 vibrates at a certain frequency to transmit an ultrasonic wave. In addition, upon receiving an ultrasonic wave, the vibrating membrane 101 vibrates. Thus, a small electric current flows in the second electrode 103 due to electrostatic induction. By measuring the value of the electric current using the transmitting and receiving circuit 302, a received signal can be retrieved. Note that while the above description has been made with reference to the DC voltage generating unit connected to the first electrode 102 and the transmitting and receiving circuit connected to the second electrode 103, a configuration in which the connections are reversed can be employed in the same manner.
An issue regarding the above-described configuration is described below with reference to FIGS. 12A and 12B. FIG. 12A illustrates the substrate 200 having a CMUT 201 mounted thereon. As an electric connection unit that connects connection electrodes 109 and 110 connected to a cell (a CMUT) 201 on a chip to the external DC voltage generating unit 301 and the transmitting and receiving circuit 302, a flexible print circuit board is used. A flexible printed circuit board is formed by forming a patterned conductive foil on an insulating film, such as a polyimide film, (hereinafter also referred to as a “base film”). The conductive foil is made of a metal, such as copper. In general, the conductive foil is about ten micrometers to several tens micrometers in thickness. The conductive foil of the base film (except for a connector portion and a conductive portion with another electrode) is covered with an insulating film, such as a polyimide film or a photo solder resist film, (hereinafter also referred to as a “coverlay”). Thus, the conductive foil is protected. Each of the base film and the coverlay is about ten micrometers to several tens micrometers in thickness. The flexible printed circuit board is several tens micrometers to a hundred and several tens micrometers in thickness. Since the flexible printed circuit board is thinner than a widely used circuit board and interconnection lines, the flexible printed circuit board is flexible and deformable.
FIG. 12B is a schematic cross-sectional view of the substrate 200 and a flexible printed circuit board 203 connected to the substrate 200 (part of a cross-sectional view taken along a line XIIB-XIIB of FIG. 12A). A connection electrode 109 disposed on the surface of the substrate 200 having a cell of a CMUT thereon and an exposed region (a flexi-side connection electrode) 141 of the conductive foil 122 of the flexible printed circuit board 203 are disposed so as to face each other. By electrically connecting the connection electrode 109 to the connection electrode 141 using an electric connecting portion 131, an electrode connected to the cell 201 can be easily connected to, for example, the external DC voltage generating unit 301 and the transmitting and receiving circuit 302. As the electric connecting portion 131, a solder bump, a gold bump, an anisotropically conductive film (ACF), or an anisotropically conductive paste (ACP), which is widely used in semiconductor flip-chip mounting, can be employed. Thus, stick-out of interconnection lines on the substrate 200 can be reduced from that in connection between the electrode of the substrate 200 and the DC voltage generating unit 301 and the transmitting and receiving circuit 302 using wire bonding.
After cells are formed on a semiconductor wafer, the wafer is cut into individual chips using a dicing saw (hereinafter, this process is also referred to “dicing”). Thus, the substrate 200 is formed. Accordingly, even when a surface of the substrate 200 (in the form of a wafer) is covered with the insulation layer 202, a semiconductor is exposed on the side surface of the substrate 200 after dicing is performed. Accordingly, if the conductive foil 122 exposed on the flexible printed circuit board 203 is brought into contact with the side surface of the substrate 200, the substrate 200 and the conductive foil (the interconnection line) 122 are short-circuited. This problem can be solved if an arrangement in which the coverlay 123 of the flexible printed circuit board 203 is placed over the substrate 200 is employed. However, in such a case, since the thickness of the coverlay 123 is significantly greater than the thickness of the connection electrode 109 and the flexi-side connection electrode 141, the flexible printed circuit board on the substrate 200 sticks out from the surface of the chip beyond the thickness of the base film 121. If the flexible printed circuit board 203 significantly sticks out, the lower limit of the thickness of the protection film may be increased in a process to form a protection film on the substrate 200. In addition, the lower limit of a distance between an acoustic lens and the chip may be increased in a process to mount the acoustic lens on the substrate 200. Thus, the transmission and reception performance may be deteriorated.